The present invention relates to capacitors mounted on standard printed circuit boards (PCBs), and in particular, to mounting pads for such capacitors.
Surface mounting of components has enabled cost and size reduction of circuit boards and enabled higher frequency operation on simple materials than was previously attainable. It is in widespread use in consumer, telecommunications, computing and many other industries.
While surface mount components are frequently used, a common alternative is components with leads. Two capacitor types are illustrated in FIGS. 1(A) and 1(B).
A common issue with leadless surface mount capacitors, particularly larger ones, is that they are prone to cracking. This can be because of mechanical stress, for example from thermal expansion, the underlying circuit board bending or from the cutting of one circuit board from the adjacent one during the manufacturing process. The older technology of leaded components suffers from this issue less because the metal leads retain a degree of flexibility that helps to relive mechanical stress that may occur.
The cracking of components jeopardizes reliability for several reasons. Firstly, because the component ceases to do its job in the circuit. It can be quite difficult to detect cracking in capacitors during manufacturing test. The second issue is that when they crack, depending upon design, the physical dislocation can cause the two plates of the capacitor to become electrically connected. As capacitors are often used for decoupling of power supplies, dislocation in this way can result in the power supply being shorted, and therefore melting, localized burning and board damage are all possible outcomes.
Capacitor cracking is a well-known problem, and has resulted in extensive literature and mitigation schemes. It typically places stringent limits on the amount of circuit board flexing allowable.
Solutions tend to fall into two categories—design changes to the surface mount component itself, and changes to the circuit board, either through the pad design or the solder used to attach to the pad.
Examples of component designs used to enable greater tolerance to mechanical stress are shown in FIGS. 2(A), 2(B), 2(C) and 2(D). FIGS. 2(A), 2(B) and 2(C) illustrate the use of leads to gain the advantage of greater flexibility. FIG. 2(D) illustrates use of an extra layer of soft material such as conductive epoxy (243) added to provide some mechanical decoupling between solder terminals and the body of the capacitor.
Another approach moves the overlap areas of the capacitor plates away from potential regions where cracking may occur. In itself this does not reduce the likelihood of cracking, but does reduce the chance of the capacitor shorting.
The methods described above add cost to the component, as well as frequently making the component larger.
The more solder that is used to connect a component to a circuit board, the better the mechanical and electrical connection. However, holding a surface mount component extremely rigidly increases the likelihood of cracking. For this reason, component manufacturers recommend the size of solder pad (FIG. 3, (302)) to use, and the amount of solder (FIG. 4, (402, 403, 404)).
These approaches are cheaper to implement than using specialized components, but also less effective—the degree of flexing that can be tolerated is lower.